Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (onmenu, choice) AS
SELECT '06,02,00,03', '06,01,00' FROM DUAL UNION ALL
SELECT '06,02,00',    '00,01,06' FROM DUAL
2 rows affected

WITH bounds (onmenu, choice, spos, epos) AS (
  SELECT onmenu,
         choice,
         1,
         INSTR(onmenu, ',', 1)
  FROM   table_name
UNION ALL
  SELECT onmenu,
         choice,
         epos + 1,
         INSTR(onmenu, ',', epos + 1)
  FROM   bounds
  WHERE  epos > 0
)
SELECT onmenu,
       choice,
       LISTAGG(
         CASE epos
         WHEN 0
         THEN SUBSTR(onmenu, spos)
         ELSE SUBSTR(onmenu, spos, epos - spos)
         END,
         ','
       ) WITHIN GROUP (ORDER BY ROWNUM) AS unchosen
FROM   bounds
WHERE  ','||choice||','
       NOT LIKE
       CASE epos
       WHEN 0
       THEN '%,' || SUBSTR(onmenu, spos) || ',%'
       ELSE '%,' || SUBSTR(onmenu, spos, epos - spos) || ',%'
       END
GROUP BY onmenu, choice


ONMENU
CHOICE
UNCHOSEN


06,02,00
00,01,06
02

06,02,00,03
06,01,00
02,03